Diaryl carbonates are useful as raw materials for the industrial production of polycarbonates that are engineering plastics. Therefore, techniques for stably producing diaryl carbonates with high efficiency are important.
A method for subjecting a dialkyl carbonate and an aromatic monohydroxy compound (for example, a phenol) to a transesterification reaction given by the following formula (A) so as to obtain an alkylaryl carbonate, and then performing a disproportionation reaction given by the following formula (B), using this alkylaryl carbonate, so as to obtain a diaryl carbonate is known as a method for producing a diaryl carbonate. The formulas (A) and (B) are as follows:
wherein R represents a linear or branched aliphatic group having 1 to 12 carbon atoms, or an alicyclic aliphatic group having 5 to 12 carbon atoms.
However, the transesterification reaction given by the above formula (A), and the disproportionation reaction given by the above formula (B) are both equilibrium reactions and have a slow reaction rate. Particularly, a problem of the transesterification reaction is that the equilibrium is biased to the original system, and therefore, the reaction efficiency is low.
In view of such problems, studies on catalysts for improving the reaction rate in the transesterification reaction given by the above formula (A), and the disproportionation reaction given by the above formula (B) have been made.
As such catalysts, for example, titanium compounds (for example, see Patent Document 1 and 2, and Non Patent Document 1), organic tin compounds (for example, see Patent Document 3, and Non Patent Document 2), nitrogen-containing basic compounds (for example, see Patent Document 4), and samarium compounds (for example, see Non Patent Document 3) have been proposed.
Further, methods for continuously producing diaryl carbonates have been also proposed. Examples of the methods include a continuous production method for separating a product from a mixture comprising a catalyst and the product by distillation and circulating the distillation residue including the catalyst component into the reaction step in order to effectively use the catalyst component (for example, see Patent Document 5 and 6). In the continuous production method, it is possible to remove part of the distillation residue including the catalyst component circulated and appropriately feed a new catalyst component, as required, for the purpose of preventing the accumulation of the by-product.
It is known that in the transesterification reaction given by the above formula (A), and the disproportionation reaction given by the above formula (B), a side reaction occurs, in addition to the reaction in which the targeted diaryl carbonate is produced. Examples of the side reaction include a Fries rearrangement reaction as given by the following formula (C). In order to separate a salicylate ester produced by the Fries rearrangement reaction, the purification of the final targeted product is essential. The formula (C) is as follows:

The separation and purification of the by-product and the targeted main product may be difficult, and purification techniques for separating the by-product have been proposed so far (for example, see Patent Document 7).
Further, when the separation and purification of the by-product and the main product is performed under high temperature conditions (for example, 200° C. or more), the catalyst function may be decreased by pyrolysis and the like. Technical proposals for methods for suppressing such a decrease in catalyst function are made (for example, see Patent Document 8).
The catalyst used in the reactions given by the above formulas (A) and (B) is solid or liquid, and there are also reports regarding methods for feeding the catalyst.
Examples of the feed methods include a method for feeding a catalyst having an alkyl moiety similar to an alkyl moiety in a dialkyl carbonate used for a raw material (for example, titanium tetraalkoxide) (for example, see Patent Document 9). According to the feed method, the mixing of the by-product of an alkyl alcohol as by-product from the catalyst can be prevented.
Other examples include a method for continuously feeding a dialkyl carbonate, an aromatic hydroxy compound, and a catalyst to a reactor, continuously removing an alcohol as by-product from a distillation column annexed to the reactor, and further removing aromatic carbonates including an alkylaryl carbonate, an diaryl carbonate, or a mixture thereof from the reactor (for example, see Patent Document 10), and a method for separating the feed position of the dialkyl carbonate and the aromatic hydroxy compound and the feed position of the catalyst (for example, see Patent Document 11). According to these methods, the precipitation of the catalyst due to operation for a long time is suppressed, and clogging can be prevented.
The catalyst used in the reactions given by the above formulas (A) and (B) is usually in a state dissolved in a reaction solution under reaction conditions. The catalyst has a higher boiling point than that of aromatic carbonates, and therefore, in order to obtain a high purity aromatic carbonate from a reaction product solution, it is necessary to first remove a low boiling component from the reaction product solution and then separate a diaryl carbonate of a high boiling component from the catalyst to purify the diaryl carbonate. It is known that at this time, the catalyst may be recovered as a high boiling component and recycled, and part of the deactivated component may be removed (for example, see Patent Document 8). A method for separating a catalyst by using a catalyst having a low boiling point, such as an alkylamine, is also known (for example, see Patent Document 12).